By Tina Mitchell Among the last species of breeding birds to arrive from their South American wintering grounds, Common Nighthawks zip and zoom through the dusk and dawn skies starting in late May and early June. A nighthawk has a big head with large eyes set on each side; its small bill opens to reveal a huge mouth; and it has short legs and weak feet. The precise coloring of nighthawks varies with the predominant color of the region’s soil: They appear grayer or buffier in Central Colorado; in the Southwest, they look more reddish brown. This cryptic coloring provides…